This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2000-75437 filed on Mar. 17, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electromagnetic driving device in which a housing portion receiving a movable element and an attracting portion attracting the movable element are integrally formed, a fluid control valve having the electromagnetic driving device and a method of manufacturing the electromagnetic driving device.
2. Description of Related Art
In a conventional electromagnetic driving device having a fixed core in which a housing portion receiving a movable element and an attracting portion attracting the movable element are formed with separate pieces, axial centers of housing portion and the attracting portion are likely to be offset due to their assembly errors. Therefore, not to disturb a reciprocal movement of the movable element even if the axial centers of the housing and attracting portions are offset to a certain extent, radial air gaps between the housing portion and the movable element and between the attracting portion and the movable element are larger by an offset amount of the axial centers thereof. Since larger air gap causes a reduction of attracting force, a greater coil winding turn is required to secure a predetermined attracting force. Increasing the coil winding turn makes a body of the electromagnetic driving device larger.
To eliminate the offset of the axial centers of the housing and attracting portions, as disclosed in JP-A-11-500509 and JP-A-7-189852, is known an electromagnetic driving device in which the housing and attracting portions are integrally formed, for example, by machining. However, when the housing and attracting portions are integrally formed, magnetic flux generated upon energizing the coil are likely to pass directly between the housing portion and the attracting portion, not through the moving element so that the attracting force is limited.
A valve disclosed in JP-A-11-500509 is provided between the housing and attracting portions with a thin thickness throttling portion having a narrow area through which magnetic flux pass for enlarging magnetic resistance. The throttling portion serves to limit the magnetic flux passing directly between the housing portion and the attracting portion so that the attracting force of the movable element may be secured.
To limit the magnetic flux passing directly between the housing and attracting portions to an extent that the attracting force is sufficiently secured, the thickness of the throttling portion has to be remarkably thin. Because of this thin thickness, a mechanical strength of the throttle portion is weak. The housing portion may thus be damaged.
Further, an electromagnetic actuator disclosed in JP-A-7-189852 is provided in a part of the housing portion with a non-magnetic material portion for enlarging the magnetic resistance. In this case, if the non-magnetic material portion is formed by diffusing the non-magnetic material, for example, by laser radiation, a surface of the non-magnetic material portion tends to be rough. As the non-magnetic portion in the actuator disclosed in JP-A-7-189852 is not in slidable contact with the moving element, the movement of the movable element is not disturbed even if an inner circumferential wall of the non-magnetic portion is rough. However, in the electromagnetic actuator in which the non-magnetic material portion comes in slidable contact with the movable element, the rough surface of the non-magnetic portion causes likely to disturb a smooth movement of the movable element.
An object of the invention is to provide an electromagnetic driving device that is compact and generates a large attracting force without damaging a housing portion of a fixed core.
It is another object of the invention to provide a fluid control valve having the electromagnetic driving device in which a driving force to a movable element is stronger without enlarging a body thereof.
Further, it is a further object to provide a method of manufacturing the electromagnetic driving device.
To achieve the above objects, in an electromagnetic driving device having a coil, a movable element, and a fixed element having a housing portion in which the movable element is housed to reciprocatingly move, and an an attracting portion toward which the movable element is attracted by magnetic flux to be generated upon energizing the coil, the housing portion is provided at a position radially opposed to the movable element with a ring portion, and the housing portion and the attracting portion are integrally formed. With the device mentioned above, the ring portion is provided with at least one of through-holes radially penetrating a wall thereof.
As the housing and attracting portions are integrally formed and worked continuously by machining. Accordingly, the offset of the axial centers of the housing and attracting portions may be eliminated. This will result in minimizing radial air gaps between the housing portion and the movable element and between the attracting portion and the movable element so that an attracting force between the attracting portion and the movable element may be increased.
Further, one or more through-holes are formed to enlarge magnetic resistance of the ring portion so that magnetic flux flowing directly between the integrally formed housing and attracting portions is limited. The magnetic resistance may be increased by adjusting a number and each diameter of the through-holes without reducing a thickness of the ring portion. Therefore, a mechanical strength of the housing portion is assured and the housing portion is prevented from being damaged.
Instead of one or more of the through-holes, the ring portion may be provided on inner or outer circumferential surface with one or more recesses or ring shaped grooves for enlarging magnetic resistance of the ring portion. The magnetic resistance may be increased by adjusting a number, each depth and largeness (width) of the recesses or ring shaped grooves without reducing a thickness of the ring portion.
Further, to enlarge magnetic resistance of the ring portion, the ring portion may be provided on an outer circumferential surface with a non-magnetic diffusion layer. Since non-magnetic material is not diffused on an inner circumferential surface of the ring portion, the inner circumferential surface of the ring portion is not rough but smooth so that a smooth slidable movement between the movable element and the ring portion may be assured.
Furthermore, to enlarge magnetic resistance of the ring portion, the ring portion is provided with a non-magnetic substance formed by inducing strains in metallographycal structure of the ring portion, for example, based on work hardening by pressing.
It is preferable that a thickness of the wall of the ring portion is thinner that that of the housing portion axially adjacent thereto to an extent that a mechanical strength of the ring portion is not damaged practically. This will contribute to enlarge magnetic resistance of the ring portion, too.
As a method of manufacturing the magnetic driving device, it is preferable that after forming one or more through-holes radially penetrating a wall of the ring portion, resin molding is executed on an outer circumferential surface of the fixed core including the ring portion, while the through-hole is closed with a blocking member abutting on an inner circumferential wall of the ring portion. As a result, resin is not leaked via the through-hole to an inside of the ring portion.
Further, it is another method that, after making a redial thickness of the ring portion thinner than that of a part of the housing portion axially adjacent thereto, resin molding is executed on an outer circumferential surface of the fixed core including the ring portion, while the ring portion is supported by a blocking member abutting on an inner circumferential wall of the ring portion so that the ring portion is prevented from deforming due to resin pressure.